The present invention is directed to the separation fo bandwidths of energy in an electro-optical device such as a spectral filter integrated into an optical demultiplexer.
Recent progress in the manufacture of optical fibers and optical waveguides with a wide low-loss spectral window of 0.8 to 1.6 .mu.m and of low threshold, long life semiconductor light sources covering the corresponding wavelength region, has made wavelength division multiplexing possible. The prior art has long recognized the advantage of optics in the transmission of a high density of information over multiple wavelengths. It is possible to have multiple transmission channels using a single optical fiber, thereby increasing the information capacity of a single optical fiber, and further realizing full duplex transmission of various types of digital and analog modulated signals. The use of such a multiplexing system, comprising a multiplexer and a demultiplexer are necessary for the transmitter and receiver, respectively. A multiplexer broadly consists of input fibers (each coupled to a source of a separate wavelength), a multiplexing circuit and a transmission fiber.
The demultiplexer consists of a transmission fiber, a demultiplexing circuit and output fibers. The multiplexing circuit couples optical signals of different wavelengths to a single transmission fiber and a demultiplexer circuit separates these optical multiple signals. An important element in the demultiplexing circuit is the capacity of separating the wavelengths for subsequent signal processing. The prior art has suggested various forms of diffraction gratings, refraction devices, prisms and thin-film filters for isolating selected bandwidths of energy. The gratings, refraction devices and prisms are angular dispersive devices, while the filters are wavelength selective devices. Generally, thin-film filters or dichroic filters have been provided in conventional optical instruments for separating one or more wavelengths such as in the video disk technology and various eye examining medical instruments.
The prior art is also aware of the advantages of miniaturized integrated optic technology wherein it is possible to fabricate a number of planar optical elements on a common substrate. Generally, such structures have the distinct advantages of compactness, ruggedness and minimal space requirements. The ability to adapt the conventional large-scale angular dispersive devices and wavelength selected devices of the conventional optic prior art is limited in the integrated optics field. The ability to deposit multilayer film upon substrate chip edge is both very difficult and an impractical solution given the fact the planar layers or waveguides of an integrated optic circuit are usually within the range of 0.2 to 2 .mu.m thick.
Thus, the prior art is still seeking to provide a broadband filter in a planar geometry for interfacing with fiber optics to perform the equivalent operation of the conventional dichroic filter.